uses-permission of "android.permission.ACCESS_WIFI_STATE" is needed in AndroidManifest.xml.

Remark on MAC:
Please take a look on the captured screen; the MAC address is 02:00:00:00:00:00.

Refer to Android 6.0 Changes - Access to Hardware Identifier:
To provide users with greater data protection, starting in this release, Android removes programmatic access to the device’s local hardware identifier for apps using the Wi-Fi and Bluetooth APIs. The WifiInfo.getMacAddress() and the BluetoothAdapter.getAddress() methods now return a constant value of 02:00:00:00:00:00.